On Fri, Jan 27, 2023 at 02:49:46PM +0100, Jules Maselbas wrote:
> Hi Peter,
>
> On Fri, Jan 27, 2023 at 12:18:13PM +0100, Peter Zijlstra wrote:
> > On Thu, Jan 26, 2023 at 06:33:54PM +0100, Jules Maselbas wrote:
> >
> > > @@ -58,9 +61,11 @@ static inline int generic_atomic_fetch_##op(int i, atomic_t *v) \
> > > static inline void generic_atomic_##op(int i, atomic_t *v) \
> > > { \
> > > unsigned long flags; \
> > > + int c; \
> > > \
> > > raw_local_irq_save(flags); \
> > > - v->counter = v->counter c_op i; \
> > > + c = arch_atomic_read(v); \
> > > + arch_atomic_set(v, c c_op i); \
> > > raw_local_irq_restore(flags); \
> > > }
> >
> > This and the others like it are a bit sad, it explicitly dis-allows the
> > compiler from using memops and forces a load-store.
> Good point, I don't know much about atomic memops but this is indeed a
> bit sad to prevent such instructions to be used.
Depends on the platform, x86,s390 etc. have then, RISC like things
typically don't.
> > The alternative is writing it like:
> >
> > *(volatile int *)&v->counter c_op i;
> I wonder if it could be possible to write something like:
>
> *(volatile int *)&v->counter += i;
Should work, but give it a try, see what it does :-)
> I also noticed that GCC has some builtin/extension to do such things,
> __atomic_OP_fetch and __atomic_fetch_OP, but I do not know if this
> can be used in the kernel.
On a per-architecture basis only, the C/C++ memory model does not match
the Linux Kernel memory model so using the compiler to generate the
atomic ops is somewhat tricky and needs architecture audits.
